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CCNA v1.1 Exam Review: Ethernet

MAC Addresses

A MAC address is a 48 bit number. The vendor code is the first 24 bits, and the last 24 bits represent a vendor assigned number. Although the 48 bit MAC addresses are written in hexadecimal, there are many ways to represent these addresses. Some examples representations are:

0000.oc12.3456
00.20.AF.12.34.56
020701123456
00-AA-12-34-56-78

Hexadecimal and Converting Between Binary and Hexadecimal

The hexadecimal system, commonly called “hex”, uses 16 symbols (as compared to the two symbols used in binary or the ten symbols used in decimal). Since we represent these symbols with only one character, and there are only 10 Arabic numbers (0 through 9), we have to use 6 new symbols to represent these numbers. We therefore borrow from the alphabet, where:

DSAP: Defines the communications pathway to the next level (network) protocol on the receiving side.
SSAP: Defines the communications pathway from the next level (network) protocol on the sending side.
Control: Defines the type of transmission.

Ethernet Cables

Unshielded twisted pair (UTP) cables for Ethernet can be either:

Straight-through cables for connecting “dis-similar” devices (such as router-switch, switch-PC or server, hub-PC or server). Straight-through cables have each wire on one end connected to the same pin on the other end.

Crossover cables for connecting “similar” devices (such as switch-switch, hub-hub, router-router, PC-PC, but also router-PC and switch-hub). Crossover cables have pin 1 on one end connected to pin 3 on the other end, and pin 2 on one end connected to pin 6 on the other end.

Though some people call a certain type of cabling “Ethernet”, it’s really a set of rules or protocols that explain how to send signals over many different kinds of cables....

Although not usually covered in training materials, it is interesting to note where Ethernet originally came from. Like many of the early networking protocols, the principles of Ethernet were developed...

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